Wood comprises two primary components, namely, a fibrous carbohydrate or cellulosic portion and a portion comprising a complex chemical, commonly referred to as lignin. For use in papermaking processes, raw wood material, such as wood chips, which may be converted into pulp fibers which may then be capable of being slurried, suspended, etc., and then deposited as a furnish on a forming screen to form a paper sheet. Such pulp fiber formation may involve physical and/or chemical treatment of the wood chips to alter its chemical form and/or to give such pulp its desired paper properties.
In chemical pulping, these wood chips may be digested with chemical solutions to solubilize a portion of the lignin and to effect the removal of such lignin. After these wood chips have been digested, the resulting cooked wood material may comprise a darkly colored pulp fiber. This darkly colored pulp fiber is commonly referred to as unbleached pulp which may be used in papermaking operations if the color of the resulting paper is unimportant. Where color of the resulting paper is relevant, this darkly colored pulp may then be bleached to increase its whiteness, brightness, etc. Bleaching of this pulp may be carried out in a single stage or in a multi-stage process involving bleaching, caustic extraction and washing operations to provide a bleached, purified and washed pulp fiber. The bleached/purified/washed pulp fiber may then be dried for use at a paper mill.
High-yield pulps may be obtained in yields of, for example, from 65 to 95% of the original weight of the wood chips used. Examples of such pulps may include refiner mechanical pulp, thermomechanical pulp, chemimechanical pulp, etc., such as chemithermomoechanical pulp (CTMP). In the manufacture of CTMP pulp, wood chips may be first impregnated with digestion chemicals and then heated (cooked) to higher temperatures (pre-cooking). These pre-cooked chips may then be defibrated in one or more disc refiners, such as a series of two disc refiners, and may also be subsequently bleached. These high-yield pulp fibers, with or without bleaching, may be used for various products, for example, as bleached fluff pulp in the manufacture of adsorbent products, as pulp fibers for paperboard, newsprint, other types of printing paper, tissue paper, etc.